1. Field of the Invention
The present invention relates to a multistage sprocket assembly for use in a bicyle, the assembly having at least two sprocket gears of different diameters.
2. Description of the Related Art
A typical multistage sprocket assembly for a bicycle is disclosed in e.g. a Japanese laid-open utility model gazette No. 64-37793. In this assembly, a width of a tooth of a small sprocket engageable with a drive chain portion to be displaced from a larger sprocket to this small sprocket is reduced relative to that of the other teeth of the small sprocket while maintaining a pressure angle of this particular tooth constant, i.e. same, relative to the other teeth.
The above-described tooth width arrangement is intended to overcome the problem associated with tension remaining in a chain segment being displaced from the large sprocket to the small sprocket. That is, although the displaced chain segment does come into engagement with the small sprocket teeth, this engagement is effected forcibly with significant driving tension still remaining in the displaced chain segment.
More particularly, the tension remaining in the displaced chain segment interferes with smooth disengagement of a trailing chain segment from the large sprocket at a proper peripheral position thereof. In the worst case, the failure of smooth and quick disengagement causes a chain jam which in turn results in damage of the derailleur due to collision between the jammed chain segment and the derailleur. The reduced tooth diameter arrangement described above is one attempt to eliminate this remaining tension.
Yet, the convention pays no attention to the pressure angle of the width-reduce tooth, which is merely maintained the same as the other teeth. More specifically, as shown in FIG. 7, the reduction of the width W1' of a tooth 15' is uniform throughout from a tooth portion adjacent a dedendum flank 16a' to a tooth crest 16b'. With this, the drive chain segment 3' as being displaced from the large sprocket 13' to the small sprocket 14' comes into deep and full engagement with the small sprocket tooth 15' down to its dedendum flank 16a'. As a result, as illustrated in FIG. 8, a longitudinal center line X' of the chain segment 3' extending between the sprockets 13' and 14' and a tangent Y' relative to the leading disengaging position on the large sprocket 13' where the chain segment 3' is about to disengage therefrom forms a small angle .beta.', whereby the chain segment 3' is improperly bent laterally. This lateral bending moment interferes with smooth and quick disengagement of the segment 3' from the large sprocket 13' as the segment 3' is strongly pressed against the side faces of the large sprocket teeth. In short, the convention is still insufficient for reliably preventing a chain jam problem.
The primary object of the present invention is to provide a further improved multistage sprocket assembly for a bicycle which improvement can substantially completely prevent a chain jam problem without entailing excessive looseness in engagement between a drive chain and a sprocket during a normal bicycle run.